1. Field of the Invention
The present invention relates to a modulator used for amplitude modulation or amplitude and phase modulation, in particular, to a modulator suitable for an integrated circuit.
2. Description of the Related Art
Nowadays, portable radio units such as portable telephone sets are being widely used. In these units, small size and low power consumption are very important factors to accomplish. In this circumference, it is desired to reduce the size and power consumption of modulation and demodulation circuits.
FIG. 30 is a block diagram showing the construction of an example of a conventional orthogonal modulator.
This orthogonal modulator comprises D/A converters 71 and 72, low-pass filters 73 and 74, a synthesizer 79, a .pi./2 phase shifter 80, mixers 77 and 78, and a composing unit 81. The D/A converters 71 and 72 each convert a digital modulation signal into an analog modulation signal. The low-pass filters 73 and 74 remove sampling noises contained in the outputs of the D/A converters 71 and 72. The synthesizer 79 generates a local signal. The .pi./2 phase shifter 80 shifts the phase of the frequency of the local signal received from the synthesizer 79 by .pi./2. The mixer 77 amplitude-modulates the output of the .pi./2 phase shifter 80 with the output of the low-pass filter 73. The mixer 78 amplitude-modulates the output of the synthesizer 79 with the output of the low-pass filter 74. The composing unit 81 composes the outputs of the mixers 77 and 78 and outputs an amplitude and phase modulation wave. The mixers 77 and 78 can be constructed of an amplitude modulator.
In the orthogonal modulator of this type, when the circuits downstream of the D/A converters 71 and 72 process analog signals, various errors and distortions take place. To reduce the distortion, a satisfactorily large bias current is required for a relevant signal. Thus, the current consumption tends to become large. This problem also takes place when a mixer is used for an amplitude modulator. In addition, an orthogonal modulator which performs a modulation with a high accuracy requires a compensation means such as a compensation circuit, a compensation signal generation circuit, or the like for compensating an error, thereby increasing the circuit scale.
In The 1991 Autumn Convention Thesis B-239 of The Institute of Electronics, Information, and Communication Engineers, a method for forming digital signals for a high accuracy orthogonal modulator is disclosed. However, in this method, digital signals with a large number of bits should be processed at high speed so as to improve the accuracy thereof. Thus, the circuit scale becomes large. In addition, the power consumption increases. In other words, this method is not suitable for portable radio terminals and so forth.
As described above, the conventional amplitude modulators or orthogonal modulators which process analog signals require satisfactorily large bias current, thereby increasing current consumption. On the other hand, modulators which form digital signals should process digital signals with a large number of bits at high speed, thereby increasing the circuit scale and power consumption.